When devised in December 1991 at The Welding Institute (TWI) in UK, many looked at Friction Stir Welding (FSW) as more of an experimental exploit confined to the bounds of laboratories. That was then.

Today, the process is spreading the tentacles of its application far and across the manufacturing world – shipbuilding, aerospace, automotive, railways, fabrication, defense, medical, electronics – you name it. After all, twenty five years is a lifetime in technological progress.

A plastic or solid state welding process, FSW does not melt materials. Instead, it heats them to the plastic stage and connects them by applying mechanical force via a welding tool. Such a technique provides high-strength, high-quality weld joints with low distortion.

In doing so, FSW sidelines the demerits of fusion welding processes incurred due to melting and solidification. Engineers choose FSW when they need sturdy joints but cannot undertake subsequent heat treatment because the welded material cannot stand such treatment.

Such materials include aluminum alloys that have seized the top spot in the list of desirables of many a designer. Blessed with high strength despite its low weight, aluminum and its alloys are replacing heavier steel – to a limited extent of course.

The Need for Friction Stir Welding (FSW)

As mentioned, aluminum and therefore its alloys are gifted with a high strength-to-weight ratio. Using such materials slashes the weight of structures. This is particularly useful for transport vehicles because it makes them burn less fuel.

Global Transport Consumes 20% of the Total Consumed Energy & Emits 22% of the Total Greenhouse Gas (GHG) Emissions – Image Courtesy of 3DDock at shutterstock.com

With their own weight thus reduced sizably, vehicles can transport greater loads while guzzling as little fuel as possible. This not only generates greater revenues, but also cuts down on fuel costs. Burning less fuel, they emit fewer pollutants.

These days, you cannot overstate the dire necessity of cutting emissions. The very survival of life on this planet depends on how efficiently we cut emissions. Because, Global Warming holds the mighty potential of flooding half the world and sucking dry the remaining half.

Global transportation gobbles up 20% of the global energy consumption. The corresponding statistic for the U.S. stands at 28%. A staggering 22% of the total global greenhouse gas (GHG) emissions can be traced to transportation with 75% of the 22% coming from road vehicles.

Welding such materials with conventional welding processes is however notoriously hard. Let us take the case of welding aluminum with traditional welding methods, an exercise beset with a host of steep challenges that include:

Hot Cracks

Stress Cracks

Porosity

Poor Penetration

Burnthrough

Discoloration

First, aluminum has high thermal conductivity and a low melting point. It conducts away the welding heat rapidly. You need to compensate by employing higher welding voltages and currents to generate more heat.

But because of its low melting point, using such high power means you have to speed up the process or risk burnthrough i.e. the formation of holes in base material. And that is not all.

If you speed up the process too much, there is lack of penetration or the weld joint not extending to the very bottom of the base materials. Striking the correct speed is a tightrope walk. Thicker sections routinely suffer from low penetration while thinner sections are exposed to burnthrough.

Then again, pure aluminum is a soft material. You have to alloy it to better its properties. But you cannot heat treat some of its alloys after welding them even though they provide a soft joint.

Next, aluminum eagerly forms oxides that restrict its melting during fusion welding. This is another reason for low penetration. The high solubility of hydrogen in molten aluminum escalates the hazard of porosity.

Finally, the low columnar strength of aluminum makes its wires vulnerable to birdnesting i.e. tangling of wire between the drive roll and liner. This limits the use of aluminum as feed metal for fusion welding. Phew! Seems welding fusion aluminum is quite a task.

The Process & Equipment Setups of Friction Stir Welding (FSW)

FSW uses a profiled probe with a broader cylinder shoulder as the tool. The control mechanism rotates the tool and feeds it along the length of the joint at a constant speed. The probe is just shorter than the required weld depth.

Being wear resistant, the tool withstands erosion as its motion generates frictional heat at the joint between the two clamped workpieces. The heat softens the workpieces. The contour of the probe is such that it forces the softened material together as it moves ahead.

The point is, FSW joins materials without melting them. A sophisticated version of forging heated materials with a hammer. By doing so, it avoids all the demerits of fusion welding that make their presence strongly felt particularly when welding aluminum and its alloys.

FSW is particularly compatible with:

long and longitudinal joints of many types viz. fillet, butt, lap, and butt-lap combination, T-butt, both side butt, butt laminate, and lap laminate

joining numerous metals with high melting points

most light metals such as aluminum, magnesium, copper, lead, zinc, and their alloys

It can easily connect 2xxx series and 7xxx series aluminum alloys conventionally considered unweldable. Robotic FSW welds 5xxx series. Copper is the major alloying metal in the 2xxx series of aluminum alloys. Zinc is the main metal in 7xxx series and magnesium in 5xxx.

And its applications are not limited to welding aluminum alloys alone. It can also weld a whole range of other metals and alloys such as titanium, high strength steels, carbon steel, and stainless steel.

Then again, FSW is capable of binding dissimilar metals into a strong joint. Not for nothing are sectors such as shipbuilding, aerospace, defense, medical, electronics, and transportation harnessing the power of this maverick process.

Designers devise the welding cycle and forces so as to minimize the possible wear, fracture, or breakage of tools and machinery

Tool Design: directly affects the strength and quality of the weld joint as well as the sped of welding because it controls the heat generation and penetration of the oxide layers on both materials

And because welding force is so important in FSW, tools must be hard, wear-resistant, tough, and strong. They must retain their hardness for long durations at elevated temperatures

Tools must also possess low thermal conductivity to minimize heat losses and thermal damage to the machinery. Plus, they must be oxidation resistant

Hot-worked tool steel such as AISI H13 has served well to weld aluminum alloys of 0.5 to 50 mm thickness. FSW of advanced materials such as metal matrix composites will require better tool material as will the FSW of diverse steels and other hard alloys

Sufficient high temperatures around the joint facilitate plastic flow for better welds by minimizing the required downward force. Excessive heat however can cause all the aforementioned defects of fusion welding

Surface Contact: is necessary to generate the correct level of friction for better quality welding. FSW apparatus with excellent downward force control give best results

Plunge Depth and Tool Tilt: plunge depth is the depth of the lowest point on the tool shoulder below the surface of the weld material. Tool tilt is the angle of the tool

Such plunging hikes pressure and provides proper welding at the rear of the tool and so does tilting the tool by 2-4 degrees in a manner that keeps the front end of the tool higher than the rear

Pros & Cons

Although an automated FSW system costs more than a conventional welding setup, it will not cost more than a cutting edge, automated laser cutting machine.

Cost however is only one parameter, not the only parameter. In order to appreciate the virtues of FSW, we need to place the entire process on to a broader canvass.

At the root of most of its qualities is the fact that FSW is a plastic welding process. Unlike fusion welding processes, it does not melt the base materials and therefore uses low heat input.

Being safer, you do not have to seek regulatory sanctions that inevitably require installation of diverse equipment and compliance with umpteen standards

More Aesthetic: joints look better than those welded by conventional processes

Other merits include:

Requires No Accessories: such as wires or shielding gas. This cuts operating costs and more than compensates for the higher capital investment

Welding from One Side: it can weld materials of between 0.5 mm and 65 mm from one side without creating voids or porosity

Automation Compatible: because its setup is simpler

But the absence of melting-solidification that imparts FSW with multiple assets also handicaps it on numerous frontiers:

Tunnel-Like Longitudinal Defects: in the weld material result from low temperature operations that render FSW incapable of accommodating large welding deformations

Kissing Defect: is the very light and insufficient contact between the weld materials. This is similar to the low penetration defect in fusion welding. Insufficient depth of the probe creates this defect

What sets off the alarm bells ringing in case of kissing defect is that you cannot detect it with ultrasonic or X-ray testing. This one’s a real silent killer

Other weaknesses of FSW include:

Requires Large Downward Forces

Leaves Exit Hole: at the location where the tool is withdrawn

Less Flexible: than arc and manual welding processes

Finally

All said and done, friction stir welding (FSW) is a process with great many capabilities that will blossom with further technological breakthroughs. And being a greener, more eco-friendly process, it will certainly command greater acceptance.

Want to know more such incredible facts and details on a host of welding process? Visit our blog.